Thanks, Jim.
> change the definitions to avoid declaring which direction is
> positive, make the direction attribute optional, and say that users
> should be careful about assuming the directionality for variables
> lacking the attribute.
This is the approach I'd prefer as well.
- Nan
Quoting Jim Biard <jbiard at cicsnc.org>:
> Nan,
>
> I didn't go to the lengths of making new regularized definitions
> before I wrote that list. I was thinking in terms of making the
> clockwise/anticlockwise call based on the right hand rule and the
> unit vector for each axis. For roll, for example, if the X unit
> vector faces forward, the "right side down" roll is actually
> anticlockwise - that is, it is in the direction that your right hand
> fingers curl if you grab the unit vector in your hand with your
> thumb pointing in the same direction as the unit vector. That
> definition is independent of observer location and look direction.
> My definitions for all the direction values are following that same
> convention.
>
> Accurate knowledge of the sign of roll, pitch, and yaw is critical
> in the satellite and aircraft world. The look angles for remote
> sensors are affected by these values. I get it that not all systems
> care about the signed values, so that reason and for backward
> compatibility I suggested that we could change the definitions to
> avoid declaring which direction is positive, make the direction
> attribute optional, and say that users should be careful about
> assuming the directionality for variables lacking the attribute.
>
> Grace and peace,
>
> Jim
>>
>> On 8/3/18 2:03 PM, Nan Galbraith wrote:
>>> Hi Roy -
>>>
>>> Yes,? I've been looking at that page quite a bit lately, and I
>>> think it backs up
>>> what I'm saying.
>>>
>>> If you are standing on that fuselage (may we never), facing
>>> forward, the red roll
>>> arrow is showing a clockwise motion, with right side moving
>>> downward. If you
>>> were facing aft, the arrow would be anticlockwise, but the right side would
>>> be rising.
>>>
>>> So, 'roll: "clockwise" for positive right side up and
>>> "anticlockwise" for positive right
>>> side down'??? - is backwards in either case. I'm not disputing
>>> anything except
>>> the term 'clockwise'? in this phrase.
>>>
>>> Thanks - Nan
>>>
>>> On 8/3/18 1:43 PM, Lowry, Roy K. wrote:
>>>>
>>>> Hi Nan,
>>>>
>>>>
>>>> Whilst I appreciate the limitations of Wikipedia as an
>>>> authoritative source have a look at
>>>> https://en.wikipedia.org/wiki/Aircraft_principal_axes
>>>>
>>>>
>>>> Cheers, Roy.
>>>>
>>>>
>>>> ------------------------------------------------------------------------
>>>> *From:* Nan Galbraith <ngalbraith at whoi.edu>
>>>> *Sent:* 03 August 2018 18:23
>>>> *To:* Jim Biard; Lowry, Roy K.; kkehoe at ou.edu
>>>> *Subject:* Re: [CF-metadata] Platform Heave (pitch, roll)
>>>> Hi Jim, Roy and Ken -
>>>>
>>>> I'm skipping the list because this is a minor point and ... and I may be
>>>> missing
>>>> something obvious.
>>>>
>>>> It's hard not to think of these terms as they apply to ships. In that
>>>> environment,
>>>> we'd use the convention of the observer facing forward; therefore roll
>>>> would be
>>>> clockwise if the right side were going down, not up. I'm supposing that
>>>> would
>>>> also apply to aircraft.
>>>>
>>>> Cheers - Nan
>>>>
>>>>
>>>>
>>>>> If we declare that X is positive forward, that Y is positive left,
>>>>> that Z is positive up, and that we use the right-hand rule for angle
>>>>> directions, the direction attribute values could be:
>>>>>
>>>>> ? * roll: "clockwise" for positive right side up and "anticlockwise"
>>>>> ??? for positive right side down.
>>>>> ? * pitch: "clockwise" for positive nose up and "anticlockwise" for
>>>>> ??? positive nose down.
>>>>> ? * yaw: "clockwise" for positive nose right and "anticlockwise" for
>>>>> ??? positive nose left.
>>>>> ? * surge: "positive" for positive forward and "negative" for
>>>>> ??? positive backward.
>>>>> ? * sway: "positive" for positive left and "negative" for positive right.
>>>>> ? * heave: "positive" for positive up and "negative" for positive down.
>>>>>
>>>>
>>>>
>>>>>
>>>>>>>
>>>>>>> ------------------------------------------------------------------------
>>>>>>> >>>
>>>>>>> *From:* CF-metadata <cf-metadata-bounces at cgd.ucar.edu> on behalf of
>>>>>>> Jim Biard <jbiard at cicsnc.org>
>>>>>>> *Sent:* 03 August 2018 15:41
>>>>>>> *To:* cf-metadata at cgd.ucar.edu
>>>>>>> *Subject:* Re: [CF-metadata] Platform Heave
>>>>>>>
>>>>>>> I freely admit that I picked direction on sway arbitrarily. In my
>>>>>>> experience, part of the variation that arises in the definitions of
>>>>>>> the different motions arises from different thoughts about their
>>>>>>> use, particularly whether someone is thinking the values are used to
>>>>>>> transform into the platform body frame vs transform from it. Or
>>>>>>> maybe they just aren't worrying about consistency. Like as not,
>>>>>>> choices have often been made in attempts to make the values have the
>>>>>>> signed-ness that felt right to people, and we can't keep to
>>>>>>> conventions like the right hand rule and make it all work
>>>>>>> consistently. We want a positive pitch to be nose up. We want a
>>>>>>> positive yaw to be nose right. We want positive heave to be up. My
>>>>>>> natural tendency is to think of "roll right" as positive and "sway
>>>>>>> right" as positive, but that isn't what other people think of.
>>>>>>>
>>>>>>> As I read what I wrote, I realize I didn't use a consistent approach
>>>>>>> to position and look direction when assigning clockwise and
>>>>>>> anticlockwise to roll, pitch, and yaw. I need to regularize that.
>>>>>>>
>>>>>>> Reading the Conventions about vertical coordinates, it says they
>>>>>>> must all have a "positive" attribute with a value of "up" or "down".
>>>>>>> I don't see a problem with having the definitions back off of
>>>>>>> declaring a specific directionality and add an attribute declaring
>>>>>>> directionality. We could call the attribute "direction" so as not to
>>>>>>> step on the "positive" attribute, and say that if the attribute is
>>>>>>> not present that the user should not assume which direction is correct.
>>>>>>>
>>>>>>> If we declare that X is positive forward, that Y is positive left,
>>>>>>> that Z is positive up, and that we use the right-hand rule for angle
>>>>>>> directions, the direction attribute values could be:
>>>>>>>
>>>>>>> ? * roll: "clockwise" for positive right side up and "anticlockwise"
>>>>>>> ??? for positive right side down.
>>>>>>> ? * pitch: "clockwise" for positive nose up and "anticlockwise" for
>>>>>>> ??? positive nose down.
>>>>>>> ? * yaw: "clockwise" for positive nose right and "anticlockwise" for
>>>>>>> ??? positive nose left.
>>>>>>> ? * surge: "positive" for positive forward and "negative" for
>>>>>>> ??? positive backward.
>>>>>>> ? * sway: "positive" for positive left and "negative" for positive
>>>>>>> right.
>>>>>>> ? * heave: "positive" for positive up and "negative" for positive down.
>>>>>>>
>>>>>>> Thoughts?
>>>>>>>
>>>>>>> BTW, I'll be out until August 13.
>>>>>>>
>>>>>>> Grace and peace,
>>>>>>>
>>>>>>> Jim
>>>>>>>
>>>
>>>
>>
>> --
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>
> --
> CICS-NC <http://www.cicsnc.org/> Visit us on
> Facebook <http://www.facebook.com/cicsnc> *Jim Biard*
> *Research Scholar*
> Cooperative Institute for Climate and Satellites NC <http://cicsnc.org/>
> North Carolina State University <http://ncsu.edu/>
> NOAA National Centers for Environmental Information <http://ncdc.noaa.gov/>
> /formerly NOAA?s National Climatic Data Center/
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Received on Fri Aug 03 2018 - 19:18:41 BST
